the bros think of her as less then human since to their eyes she does not have the sword talent to be a knight, and that is all that matters to them. There is a side chapter in the WN from the younger bros perspective of FIA returning, and :
a) they are only there due to the big sis
b) his thoughts are it would be better she
did not return as it would mean they waste less food
Their actions in the next 2 chapters are horrible, both are acting as examiners for the knight exam, and their performance there can only be described as incompetence.